The present invention relates to an alloyed hot-dip galvanized steel sheet with excellent sliding property which is suitable as an anti-corrosive steel sheet for use in car bodies.
Alloyed hot-dip galvanized steel sheets are widely used as anti-corrosive steel sheets for use in car bodies. The alloyed hot-dip galvanized steel sheets are molded into car bodies by pressing and must therefore be excellent not only in anti-corrosive properties but also in sliding properties.
Process steps for manufacturing an alloyed hot-dip galvanized steel sheet are broadly divided into the process step of immersing a material steel sheet in a plating bath to form a hot-dip galvanized layer on the surface of the steel sheet, and the step of subjecting the steel sheet carrying the hot-dip galvanized layer to alloying to thereby form an alloyed hot-dip galvanized layer.
The hot-dip galvanized layer formed on the surface of the steel sheet in the plating bath comprises an intermetallic compound of Zn and Fe (xcex6, xcex41, xcex93), and the sliding property of the alloyed hot-dip galvanized layer formed by alloying varies depending on the composition of the intermetallic compound. Various techniques have therefore been proposed in which the sliding property of an alloyed hot-dip galvanized steel sheet is improved by controlling the composition of such an intermetallic compound of the hot-dip galvanized layer formed prior to alloying.
For example, Japanese Unexamined Patent Application Publication No. 9-209106 discloses a steel sheet for use in alloyed hot-dip galvanized and an alloyed hot-dip galvanized steel sheet. This technique intends to form an alloyed hot-dip galvanized layer with satisfactory sliding property by controlling the composition of the base steel sheet. However, the composition of the hot-dip galvanized layer varies with changes in operating conditions of the plating process step and affects the sliding property of the resulting alloyed hot-dip galvanized layer. Accordingly, the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-209106 cannot significantly yield satisfactory sliding property stably.
Japanese Unexamined Patent Application Publication No. 11-200004 discloses an alloyed hot-dip galvanized steel sheet with excellent sliding property. This technique intends to manufacture an alloyed hot-dip galvanized steel sheet with high sliding property and resistance to plating adhesion failure by depositing a crystal mainly containing Ti and Al on the surface of the alloyed hot-dip galvanized layer. To deposit a crystal mainly containing Ti and Al, a plating bath must comprise Ti. However, when the plating bath comprises Ti, a Tixe2x80x94Al intermetallic compound (so-called xe2x80x9cdrossxe2x80x9d) is formed and adheres to the hot-dip galvanized layer to cause problems in the surface appearance of the resulting galvanized steel sheet.
Accordingly, an object of the present invention is to solve the above problems and to provide an alloyed hot-dip galvanized steel sheet stably exhibiting excellent sliding property.
The present inventors have made detailed investigations on alloyed hot-dip galvanized layers with excellent sliding property.
Specifically, the present inventors have made investigations on-electrolytic behavior of alloyed hot-dip galvanized layers by electrolyzing them according to a constant potential electrolysis using various alloyed hot-dip galvanized steel sheets as an anode and an aqueous zinc sulfate-sodium chloride solution as an electrolyte. Furthermore, the present inventors have made investigations on the relationship between the quantity of electricity required by the constant potential electrolysis and the sliding property. As a result, they have found that alloyed hot-dip galvanized steel sheets exhibiting a potential of less than or equal to a specific level, when they are immersed in the electrolyte, have satisfactory sliding property and that alloyed hot-dip galvanized steel sheets, in which the total quantity of electricity consumed until the completion of electrolysis is less than or equal to a specific level, have satisfactory sliding property.
The present invention has been accomplished based on these findings and further investigations.
Specifically, the invention provides an alloyed hot-dip galvanized steel sheet with excellent processability and particularly with excellent sliding property, exhibiting a potential to a saturated calomel electrode of less than or equal to xe2x88x92850 mV when it is immersed in a zinc sulfate-sodium chloride electrolyte. The invention also provides an alloyed hot-dip galvanized steel sheet with excellent processability and particularly with excellent sliding property in which, when the alloyed hot-dip galvanized steel sheet is electrolyzed according to a constant potential electrolysis process in a zinc sulfate-sodium chloride electrolyte at a potential to a saturated calomel electrode of from xe2x88x92940 mV to xe2x88x92920 mV, the quantity of electricity consumed is less than or equal to 0.5 C/cm2.
When the quantity of the electricity is less than or equal to 0.3 C/cm2, the resulting alloyed hot-dip galvanized steel sheet exhibits preferable sliding property.